Third-generation CT scanners typically include an x-ray source and an array of x-ray detectors secured respectively on diametrically opposite sides of an annular disk, the latter being rotatably mounted within a gantry support. During a scan of a patient located within the opening of the disk, the disk rotates about a rotation axis while x-rays pass from the focal spot of the X-ray source through the patient to the detector system.
The x-ray source and detector array are positioned so that the x-ray paths between the focal spot and each detector all lie in the same plane (the so-called "slice plane", "rotation plane" or "scan plane") which is normal to the rotation axis of the disk. Because the ray paths originate from substantially a point source and extend at different angles to the detectors, the ray paths resemble a fan, and thus the term "fan beam" is used to describe all of the ray paths at any one instant of time. The term "sub-beam", as used herein, refers to the radiation that is detected by a single detector at a measuring instant during a scan. The sub-beam is partially attenuated by the mass of the patient in its path, and each detector generates a single intensity measurement as a function of the attenuation, and thus of the density of the portion of the patient in the path of the sub-beam from the focal spot to that detector. These x-ray intensity measurements, or projections, are typically performed during prescribed measurement intervals at each of a plurality of angular disk positions.
The term "radial", as used herein, refers to a direction in the scan plane from or toward the focal spot of the x-ray source. The term "tangential", as used herein, refers to a direction in the scan plane which is substantially perpendicular to the radial direction.
Various types of detectors have been developed, including gas and solid state types. A typical solid state detector includes a scintillating crystal which converts high energy x-radiation photons into low energy visible light photons, and a photodiode which converts the low energy visible light photons into extremely low-amplitude electrical currents (i.e., on the order of picoamperes to nanoamperes). The output of each detector represents the x-ray flux incident on the detector. The outputs of the detector array are transmitted via an array of conductors to a data acquisition system (DAS) for signal processing.
A typical gas detector comprises a pressure vessel with a radiation-translucent, or radiolucent, window which permits x-rays to enter the vessel with a minimum of attenuation. Within the vessel are a large number of electrically conductive plates of non-uniform dimension or nonuniform interplate spacings which are surrounded by a high-pressure inert gas, such as xenon, which is ionized by the x-rays. The conductive plates define discrete regions which are swept by electric fields. A region which supplies electrical charge to a particular plate defines a single sub-beam of radiation. The conductive plates can be radially oriented with respect to the axis of rotation of the scanner, thereby providing electric fields in the tangential direction. Alternatively, the plates can be oriented perpendicular to the rotation axis to provide electric fields in the axial direction.
The width in the tangential direction of a single detector (hereinafter, "detector" or "detector crystal") defines the width of a single sub-beam of radiation emitted from the x-ray source. Because resolution of the resulting image is a function of the width and spacing of the detectors, a CT scanner system typically includes hundreds of relatively small detectors which are extremely closely spaced along an arc of a circle extending about the focal spot. For example, one third-generation CT scanner system manufactured by the present assignee includes 384 detectors provided by 24 modules of 16 detectors each and closely spaced within a 48-degree arc extending about the focal spot. The width of a single detector in such prior art systems is thus on the order of a millimeter.
In so-called fan beam tomography, radiation is emitted in a fan-shaped beam from the x-ray source, and data about the object being scanned is obtained from a series of fan-shaped projections, or views, taken at constant angular increments about the patient. However, the complexity of the computations required to reconstruct images from the data can be significantly reduced if the fan beam data is grouped in sets of parallel beam data and the image reconstructed using parallel beam algorithms. To accomplish this, data from parallel rays from different projections are grouped together into sets. The parallel beam data sets are then interpolated so that the distance between adjacent sub-beams is a constant value. More specifically, interpolation of the parallel beam data sets ensures that the distance from the isocenter to successive adjacent sub-beams is a constant increment of the radius of the circle on which the detectors are located. This constant spacing between adjacent sub-beams is required for accurate image reconstruction using parallel beam convolution and back-projection methods.
A disadvantage of this technique is that interpolation of data is difficult and time-consuming, and the data obtained thereby is inherently of lower spatial resolution. It is therefore preferable to sample and obtain data substantially as it will be used in the reconstruction of an image, instead of obtaining data which must be significantly interpolated in order to approximate the information required to reconstruct images.
In prior art CT systems, the detectors have typically been spaced at equal angular increments along a circle centered at the focal spot. However, this detector configuration is not necessarily the most desirable configuration, as the radial extent of the detectors in this configuration may be relatively great, leaving little room for the patient and for other necessary components, including power sources and cables. Although the entire system could be made larger to accommodate such a detector configuration, it is preferred to make the system more compact and thus more convenient and cost-effective for the user.
It would therefore be advantageous to provide an x-ray scanning system which overcomes the limitations of the prior art systems.